iOS only: Boxcar already did a great job of acting as a great universal push notification app, but now it's even better. The app supports Google Voice and Buzz notifications and offers a new dashboard for choosing how each app notifies you.
In other words, you can now choose how often and how dramatic the notifications from your RSS feeds, email, Twitter and other services show up. For some notifications, you can also reply directly from the Boxcar app. There's been a redesign too, and support for iPhone 4 Retina displays is built in.
Boxcar is a free app with advertising and available for $5.99 as an ad-free purchase for iOS devices.